The State Department should deny the permit on the grounds that the line could put the Ogallala Aquifer at risk, the Midwestern state's governor, Dave Heineman, said in a letter to the president and Secretary of State Hillary Clinton.

The State Department said in last week's environmental review that an oil spill from the line would affect a limited area in Nebraska's Sand Hills region, which is part of the High Plains aquifer.

Heineman said he does not oppose pipelines in general, but disagreed about the risks.

"This resource is the lifeblood of Nebraska's agriculture industry," he said in the letter. "I am concerned that the proposed pipeline will potentially have detrimental effects on this valuable natural resource and Nebraska's economy."

Nebraska Senator Mike Johanns echoed Heineman's request. "The proposed route is the wrong route. It's clear to me, after traveling throughout the state, that most Nebraskans agree a better route is needed," Johanns said in a release.

The opposition of the Nebraska politicians lends support to a growing movement to stop the pipeline altogether because of concerns that mining crude from oil sands will sharply increase greenhouse gas emissions.

Nearly 600 opponents of the line have been arrested in front of the White House in an action set to last until the weekend. It is unclear what opponents can do to stop the project, which TransCanada hopes to have built by 2013.

Montana's Governor Brian Schweitzer has supported the Keystone project, in part because the line could help relieve a buildup of oil from his state as drilling techniques open access to new supplies.

(Reporting by Timothy Gardner in Washington and Joshua Schneyer in New York; Editing by Dale Hudson)